The present invention relates generally to integrated circuit memory devices and, more particularly, to an apparatus and method for small signal sensing in static random access memory (SRAM) cells utilizing PFET access devices.
A typical static random access memory (SRAM) cell includes an array of individual SRAM cells. Each SRAM cell is capable of storing a binary voltage value therein, which voltage value represents a logical data bit (e.g., “0” or “1”). One existing configuration for an SRAM cell includes a pair of cross-coupled devices such as inverters. With CMOS (complementary metal oxide semiconductor) technology, the inverters further include a pull-up PFET (p-channel) transistor connected to a complementary pull-down NFET (n-channel) transistor. The inverters, connected in a cross-coupled configuration, act as a latch that stores the data bit therein so long as power is supplied to the memory array.
In a conventional six-transistor cell 100 such as shown in FIG. 1, a pair of access transistors or pass gates T1, T2 (when activated by a word line LWL) selectively couples the inverters to a pair of complementary bit lines BLT, BLC. Prior to a read operation, the bit lines BLT, BLC are precharged to the power supply voltage VDD. A read operation commences when a restore circuit (not shown) is turned off, and the word line LWL is driven high so as to activate NFET pass gates T1, T2. This in turn electrically connects the internal nodes A, B of the cell 100 to bit lines BLT, BLC, respectively. Whichever of the two bit lines is connected to the “low” (logic 0) cell node will begin to discharge to ground at a rate proportional to the current drive of the cell (i.e., the series connection of either T2 and T4, or T1 and T3), and the capacitance of the bit lines. The bit line connected to the “high” (logic 1) cell node will be left floating high, since its respective word line access device is cut off (i.e., both the drain and source terminals thereof are at VDD potential). The voltage difference between the bit line discharging to ground and the bit line left floating high is referred to as the bit line signal. Because the bit lines are typically highly capacitive, and cell current is typically relatively low, it generally takes a significant amount of time to generate an adequate amount of signal to reliably sense and amplify the cell data.
Accordingly, it would be desirable to be able to accurately sense the data in an SRAM cell at smaller level of signal differential than a conventional cell, and thus at an earlier time in the read cycle.